Understanding HoloLens Spatial Mapping and Hologram Ranges

There seems to be some confusion over what the HoloLens’s depth sensor does, on the one hand, and what the HoloLens’s waveguides do, on the other. Ultimately the HoloLens HPU fuses these two two streams into a single image, but understanding the component pieces separately are essential for creating good holographic UX.


From what I can gather (though I could be wrong), HoloLens uses a single depth camera similar to the Kinect  to perform spatial mapping of the surfaces around a HoloLens user. If you are coming from Kinect development, this is the same thing as surface reconstruction with that device. Different surfaces in the room are scanned by the depth cameras. Multiple passes of the scan are stitched together and merged over time (even as you walk through the room) to create a 3D mesh of the many planes in the room.


These surfaces can then be used to provide collision detection information for 3D models (holograms) in your HoloLens application. The range of the depth spatial mapping cameras is 0.85 meters to 3.1 meters. This means that if a user wants to include a surface that is closer than 0.85 M in their HoloLens experience, they will need to lean back.

The functioning of the depth camera shouldn’t be  confused with the functioning of the HoloLens’s four “environment aware” cameras. These cameras are used to help in nailing down the orientation of the HoloLens headset in what is known as  inside-out positional tracking. You can read more about that in How HoloLens Sensors Work. It is probably the case that the depth camera is used for finger tracking while the 4 environment aware cameras are devoted to spatial mapping.

The depth camera spatial mapping in effect creates a background context for the virtual objects created by your application. These holograms are the foreground elements.

Another way to make the distinction based on technical functionality rather than on the user experience is to think of the depth camera surface reconstruction data as input , and holograms as output. The camera is a highly evolved version of the keyboard while the waveguide displays are modern CRT monitors.

It has been misreported that the minimum distance for virtual objects in HoloLens is also 0.85 Meters. This is not so.

The minimum range for hologram placement is more like 10 centimeters. The optimal range for hologram placement, however, is 1.25 m to 5 m. In UWP, the ideal range for placing a 2D app in 3D holographic space is 2 m.

Microsoft also discusses another range they call the comfort zone for holograms. This is the range where vergence-accommodation mismatch doesn’t occur (one of the causes of VR sickness for some people). The comfort zone extends from 1 m to infinity.

Range Name Minimum Distance Maximum Distance
Depth Camera 0.85 m (2.8 ft) 3.1 m (10 ft)
Hologram Placement 0.1 m (4 inches) infinity
Optimal Zone 1.25 m (4 ft) 5 m (16 ft)
Comfort Zone 1.0 m (3 ft) infinity

The most interesting zone, right now, is of course that short range inside of 1 meter. That 1 meter min comfort distance basically prevents any direct interactions between the user’s arms and holograms. The current documentation even says:

Assume that a user will not be able to touch your holograms and consider ways of gracefully clipping or fading out your content when it gets too close so as not to jar the user into an unexpected experience.

When a user sees a hologram, they will naturally want to get a closer look and inspect the details. When a human being looks at something up close, he typically wants to reach out and touch it.


Tactile reassurance is hardwired into our brains and is a major tool for human beings to interact with the world. Punting on this interaction, as the HoloLens documentation does, is a great way to avoid this basic psychological need in the early days of designing for augmented reality.

We can pretend for now that AR interactions are going to be analogs to mouse click (air-tap) and touch-less display interactions. Eventually, though, that 1 meter range will become a major UX problem for everyone.

[Updated 4/6 after realizing I probably have the functioning of the environment aware cameras and the depth camera reversed.]

[Updated 4/21 – nope. Had it right the first time.]

HoloLens Occlusion vs Field of View


[Note: this post is entirely my own opinion and purely conjectural.]

Best current guesses are that the HoloLens field of view is somewhere between 32 degrees and 40 degrees diagonal. Is this a problem?

We’d definitely all like to be able to work with a larger field of view. That’s how we’ve come to imagine augmented reality working. It’s how we’ve been told it should work from Vernor Vinge’s Rainbow’s End to Ridley Scott’s Prometheus to the Iron Man trilogy – in fact going back as far as Star Wars in the late 70’s. We want and expect a 160-180 degree FOV.

So is the HoloLens’ field of view (FOV) a problem? Yes it is. But keep in mind that the current FOV is an artifact of the waveguide technology being used.

What’s often lost in the discussions about the HoloLens field of view – in fact the question never asked by the hundreds of online journalists who have covered it – is what sort of trade-off was made so that we have the current FOV.

A common internet rumor – likely inspired by a video by tech evangelist Bruce Harris taken a few months ago – is that it has to do with cost of production and consistency in production. The argument is borrowed from chip manufacturing and, while there might be some truth in it, it is mostly a red herring. An amazingly comprehensive blog post by Oliver Kreylos in August of last year went over the evidence as well as related patents and argued persuasively that while increasing the price of the waveguide material could improve the FOV marginally, the price difference was prohibitively expensive and ultimately nonsensical. At the end of the day, the FOV of the HoloLens developer unit is a physical limitation, not a manufacturing limitation or a power limitation.


But don’t other AR headset manufacturers promise a much larger FOV? Yes. The Meta 2 (shown below) has a 90 degree field of view. The way the technology works, however, involves two LED screens that are then viewed through plastic positioned at 45 degrees to the screens (technically known as a beam splitter, informally known as a piece of glass) that reflects the image into the user’s eyes at approximately half the original brightness while also letting the real world in front of the user (though half of that light is also scattered). This is basically the same technique used to create ghostly images in the Haunted Mansion at Disneyland.


The downside of this increased FOV is you are loosing a lot of brightness through the beam splitter. You are also losing light based on the distance it takes the light to pass through the plastic and get to your eyes. The result is a see-through “hologram”.


But is this what we want? See-through holograms? The visual design team for Iron man decided that this is indeed what they wanted for their movies. The translucent holograms provide a cool ghostly effect, even in a dark room.


The Princess Leia hologram from the original Star Wars, on the other hand, is mostly opaque. That visual design team went in a different direction. Why?


My best guess is that it has to do with the use of color. While the Iron Man hologram has a very limited color palette, the Princess Leia hologram uses a broad range of facial tones to capture her expression – and also so that, dramatically, Luke Skywalker can remark on how beautiful she is (which obviously gets messed up by the Return of the Jedi). Making her transparent would simply wash out the colors and destroy much of the emotional content of the scene.


The idea that opacity is a pre-requisite for color holograms is confirmed in the Star Wars chess scene on the Millennium Falcon. Again, there is just enough transparency to indicate that the chess pieces are holograms and not real objects (digital rather than physical).


So what kind of holograms does the HoloLens provide, transparent or near-opaque? This is something that is hard to describe unless you actually see it for yourself but the HoloLens “holograms” will occlude physical objects when they are placed in front of them. I’ve had the opportunity to experience this several times over the last year. This is possible because these digital images use a very large color palette and, more importantly, are extremely intense. In fact, because the holoLens display technology is currently additive, this occlusion effect actually works best with bright colors. As areas of the screen become darker, they actually appear more transparent.

Bigger field of view = more transparent , duller holograms. Smaller field of view = more opaque, brighter holograms.

I believe Microsoft made the bet that, in order to start designing the AR experiences of the future, we actually want to work with colorful, opaque holograms. The trade-off the technology seems to make in order to achieve this is a more limited field of view in the HoloLens development kits.

At the end of the day, we really want both, though. Fortunately we are currently only working with the Development Kit and not with a consumer device. This is the unit developers and designers will use to experiment and discover what we can do with HoloLens. With all the new attention and money being spent on waveguide displays, we can optimistically expect to see AR headsets with much larger fields of view in the future. Ideally, they’ll also keep the high light intensity and broad color palette that we are coming to expect from the current generation of HoloLens technology.

HoloLens Hardware Specs


Microsoft is releasing an avalanche of information about HoloLens this week. Within that heap of gold is, finally, clearer information on the actual hardware in the HoloLens headset.

I’ve updated my earlier post on How HoloLens Sensors Work to reflect the updated spec list. Here’s what I got wrong:

1. Definitely no internal eye tracking camera. I originally thought this is what the “gaze” gesture was. Then I thought it might be used for calibration of interpupillary distance. I was wrong on both counts.

2. There aren’t four depth sensors. Only one. I had originally thought these cameras would be used for spatial mapping. Instead just the one depth camera is, and it maps a 75 degree cone out in front of the headset, with a range of 0.8 M to 3.1 M.

3.  The four cameras I saw are probably just grayscale cameras – and it’s these cameras along with cool algorithms that are being used to do inside-out position tracking along with the IMU.

Here are the final sensor specs:

  • 1 IMU
  • 4 environment understanding cameras
  • 1 depth camera
  • 1 2MP photo / HD video camera
  • Mixed reality capture
  • 4 microphones
  • 1 ambient light sensor

The mixed reality capture is basically a stream that combines digital objects with the video stream coming through the HD video camera. It is different from the on-stage rigs we’ve seen which can calculate the mixed-reality scene from multiple points of view. The mixed reality capture is from the user’s point of view only. The mixed-reality capture can be used for streaming to additional devices like your phone or TV.

Here are the final display specs:

  • See-through holographic lenses (waveguides)
  • 2 HD 16:9 light engines
  • Automatic pupillary distance calibration
  • Holographic Resolution: 2.3M total light points
  • Holographic Density: >2.5k radiants (light points per radian)

I’ll try to explain “light points” in a later post – if I can ever figure it out.